It is generally recognized that the performance of a steam turbine is greatly influenced by last stage bucket designs that efficiently utilize the expansion of steam to the exhaust pressure of the site, and that minimize the kinetic energy of the flow leaving the last stage bucket.
The service requirements of turbine buckets, however, can be very complex and demanding. This is particularly true in the case of last stage steam turbine buckets which experience a variety of conditions including corrosive environments due to high moisture and carry over from the boiler. These conditions lead to various types of corrosion of the bucket material. Moreover, turbine buckets also experience high tensile loadings and are subject to cyclic stresses which, when combined with a corrosive environment, can be particularly damaging to the bucket. In addition, since the steam is very wet in the last stage region, water droplet erosion of the bucket material occurs which, in turn, reduces the useable service life of the bucket as well as the efficiency of the turbine as a whole. In advanced cases of erosion, actual failure of the bucket may occur.
It is difficult and, in some cases impossible, to find a bucket material which can meet all of the required critical properties in a particular design application. This is especially true in advanced designs where longer buckets are specified, increasing both the strength requirements and the severity of erosion experienced by the buckets. Moreover, the higher stresses inherent in these designs increases the potential for stress corrosion cracking, and the higher strength required in the bucket material further exacerbates the problem through increased susceptibility to such cracking. The effects of pitting corrosion in initiating corrosion fatigue is also magnified by the higher applied stresses. Thus, alloy selection to satisfy basic bucket requirements may not be compatible with erosion resistance requirements, nor the attachment of erosion resistant shielding material.
Previous approaches to solving this problem depended on specific requirements. In some cases, where demands were not so servere, one material in one condition could satisfy all the requirements. When greater erosion resistance was required, the bucket material has been hardened through local heat treating (flame or induction hardening) at the leading edge to provide additional erosion resistance. Alternatively, an erosion resistant shielding material such as stellite has been attached (by brazing, gas tungsten arc or electron beam welding) at the near finished machined stage in the bucket production. The attachment methods almost invariably lead to some degradation of properties in the weld heat affect zone, however, and defective welds resulted in costly scrapping of finished buckets.
In published European patent application 0 379 922, there is disclosed a method of manufacturing or repairing a turbine blade which includes welding an insert to a leading edge of the blade and then hardening a part of the insert so as to provide a leading outer edge part of the blade with a hardened surface. The disclosure is careful to point out that the hardening does not fully extend as far as the junction between the insert and the blade so that there exists adjacent the boundary with the remainder of the blade a portion of the insert of substantially unhardened material.